Drilling for oil or gas is a complicated and dangerous endeavor where drillers must blend both technology and experience to create a working well. The early stages of the well development are the most critical and due to the lack of blow-out protection, it is also the most hazardous period for the crew working the rig.
Setup for drilling an oil or gas well will vary depending upon location and ground conditions. However, a typical surface drilling operation would begin by preparing the site and grading the drill pad. Once the drill pad is leveled, the location for the conductor and cellar ring is determined. The cellar is typically a five foot to twelve foot diameter corrugated steel culvert pipe that is set into the ground. The cellar provides clearance for the diverter lines below the blow out protection, or BOP, and is available to control leakage and spills around the well bore. The cellar hole may be bored using a large diameter auger or simply dug out and refilled around the outside of the culvert pipe using an excavator. The next step is to set a relatively short string of large diameter conductor pipe. The conductor pipe will act as a drilling guide and prevents unconsolidated surface material from collapsing into the conductor hole. The conductor pipe averages 120 feet long for an on surface development, or a much longer string in an offshore drilling application. Where the surface material has allowable cohesive properties, the operator will first drill an oversized hole using an auger prior to inserting the conductor pipe into the hole. In other applications the conductor string may be driven to the desired depth. After being set to the desired depth, the space between the outside of the conductor pipe wall and original hole is cemented. Once the conductor pipe is in place and the cement has set, the operator will prepare and level the pipe end by cutting and/or grinding.
With the cellar and conductor string in place, the drill rig can now be moved onto location, centering the derrick over the cellar ring. The substructure, or subframe, typically elevates the subfloor, rotary beams, and table 25 to 50 feet above the ground. The derrick extends above the subframe.
The process of leveling and positioning the top of the conductor pipe with the subfloor is called “nippling up.” Nippling up is time consuming, typically requiring 12 to 14 hours, and exposes the crew and equipment to hazardous working conditions. The first step is to determine the actual length of conductor pipe needed; this is accomplished typically by a welder extending a measuring tape from the subfloor to the top of the conductor pipe in the cellar. The measurement needs to be fairly accurate but will likely occur under adverse conditions such as the welder hanging over a 50 foot opening in the subfloor and the flexible tape being blown around by the wind. Once the length is determined, a suitable section of conductor pipe is selected from the parts tub; the pipe will likely have numerous welds and dents from previous installations. The conductor pipe may or may not have a flow nipple. If there is no flow nipple, the welder will cut a 10 or 12 inch hole in side of the conductor pipe near one end, grind and prepare the hole, shape a short section of reciprocal pipe that will be welded over the hole, perpendicular to the conductor pipe.
The flow nipple end of the conductor pipe is then attached to a winch or “air tugger” on the derrick and the base end is attached, using a sling or cable, to a piece of heavy equipment, such as a loader or excavator. In order to move the conductor pipe under the subframe, the operators must tension the slings using the air tugger to suspend the pipe horizontally and then slowly walk the equipment forward while maintaining tension with the air tugger and elevating the conductor pipe toward the rotary head. Once the conductor pipe is pulled under the subframe, the bottom sling is released and roughnecks on the ground will manually position the pipe over the conductor string in the ground and hold it in place. Additional manpower or rough necks are positioned in the cellar ring and will be responsible for aligning and holding the flow nipple position. Once alignment is achieved, a welder will tack the two sections of conductor pipe together before the roughnecks can release the pipe and the welder can complete the weld. The welder will then cut a hole near the base of the conductor and weld in a ¾ inch threaded collar or “threadolet”, used to check for concrete returns when the drill casing is cemented in place.
If the conductor pipe is too long, the conductor stand will be removed from under the subframe, the welder will cut it down, and the fitting and welding process will start over. If the conductor pipe is too short, the conductor pipe is removed and an additional section of pipe is welded into place. With the conductor pipe completed, the rig can now start drilling. However, this drilling is without any blowout protection in place.
The next step is to “drill for surface” or drill a smaller hole through the conductor pipe, preparatory for setting the surface casing. The depth of the surface casing will vary depending on the depth of the ultimate target, ground conditions, surface anomalies and other factors. However, this step may be the most perilous; and the shallower the well bore, the greater the danger a surface anomaly will be to the men and equipment. During the drilling process it is common for the well bore to intercept pressurized water, as well as, gas pockets of methane or hydrogen sulfide in the rock or the coal formation being penetrated; the gases can be under extremely high pressure and are flammable. Drilling fluid or drilling “mud” is a mixture of water, clay and myriad other ingredients; the mud is injected under pressure down the center of the drill string, and as the mud recirculates back to the surface, on the outside of the drill string, the mud carries the rock cuttings out of the hole and serves to condition and seal the walls of the bore. Additionally, a driller may adjust the specific density of the drilling mud to increase hydrostatic pressure and help to control the flow of formation gas to the surface. However, if gas is encountered at shallow depth and the hydrostatic pressure of the drilling fluid in the vertical bore column is insufficient to overcome the pressure of the formation gas, the gas will blowout the top of the conductor pipe at high pressure and volume, potentially causing equipment damage and injury, and if the gas is flammable, the danger of an explosion and fire is also present.
Once “surface” has been set, a casing string will be inserted, through the conductor, into the surface hole and cemented into place. The cement is pumped down the inside of the casing under high pressure and will return to the surface outside of the casing; filling the voids between the casing string and the earth wall of the well bore, and the space between the casing and the conductor as the concrete reaches the surface. The casing is full when wet cement appears in the ¾ inch treaded collar at the base of the conductor. The operation must now wait until the cement has cured.
Once the casing cement has cured, the welder must now cut 3 to 4 inspection holes or working holes in the side of the conductor, and a rough cut to free the top portion of the casing. This operation is perilous. If the surface hole has intercepted even a low pressure gas pocket, sparks from cutting the casing may ignite the gases. It is typical for the welder to make the cuts while keeping his head and body below the inspection holes and limit exposure to the arm and hand.
The concrete between the conductor and the casing is visually inspected in order to see if the concrete has cured properly and is still adequately full around the casing. However, it is possible that during the curing time the concrete, that was once flowing out of the collar at the base of the conductor, has breached an anomaly in the formation which allowed the concrete to slump or “fall back” below the top of the casing. If there are indications of problems, the conductor pipe must completely cut and slung out from under the drilling subframe using the reverse procedure described above for bringing it in. The concrete level between the wall of the casing and conductor pipe is inspected visually and distance measurements may be taken by inserting a flexible tape down the conductor; if the concrete has fallen back, the resulting void must be refilled. However, since the concrete has now cured, you can no longer introduce concrete down the casing string and the concrete must be added from the surface in a procedure referred to as a “top job”. When doing a top job, concrete must be ordered, brought to the drill site, and discharged directly down the space between the casing and the conductor. During this operation all personnel are exposed to the risks of what every type of anomaly the concrete fell back in to.
Once the casing is cemented in place, the casing can then be fitted with a BOP or blowout preventer. The BOP, as commonly known in the art, may be a ram or shear arrangement that blocks the well bore in the case of escaping gas or fluid. However, a BOP will not protect the drilling platform and crew until the surface casing is installed and a BOP installed on the surface casing string cannot protect the crew in the situation of a “behind casing blowout”.
A behind casing blowout occurs when gas escapes towards the drill platform between the casing and conductor. This may occur because the pressure wave from the escaping gas shocks the casing wall, fracturing or separating the cement between the casing and the conductor. It is also postulated that the differential of thermal expansion between the steel and cement creates separations or channels where high pressure gas can pass through.
What is needed is a device that can be quickly installed during the conductor nipple up process; that will protect the drilling platform and crew during the surface drilling stage, and will protect the drilling platform and crew during a surface blow out or during a top job.